Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
In particular, batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. Other metals, such as Zn, Mg, Na, Fe, Al, Ca, Si, and others, also have a favorable specific energy and energy density.
To enable electric vehicles with a range approaching that of present-day vehicles (>300 miles) a battery chemistry with a significantly higher specific energy is required than the present state of the art lithium-ion batteries. The Department of Energy has set a long-term goal for the maximum weight of an electric vehicle battery pack to be 200 kg (this includes the packaging). The use of other metals can also offer a higher specific energy than Li-ion cells with conventional positive electrodes. Through the use of a lithium metal negative electrode and a positive electrode reacting oxygen, a driving range above 300 miles is possible. A driving range above 300 miles may also be possible with other metals.
The lithium-sulfur (Li/S) battery chemistry is attractive due to its high theoretical gravimetric energy density (2600 Wh/kg) and low cost of the active cathode material, sulfur. Typical Li/S cells involve solid charge and discharge products (S8 and Li2S [or Li2S2], respectively) that undergo conversion to soluble polysulfides (Li2Sn, 2<n<=8) at intermediate degrees of lithiation during the charge and discharge processes.
There are significant challenges that must be addressed for the lithium-sulfur system to become commercially viable. Important challenges include increasing the cycle life (current state of the art is 100 to several hundred cycles; target is >500, preferably >2000), increasing the utilization of sulfur (typical utilization is below 75% due to passivation of the positive electrode by Li2S or Li2S2, which are electronically insulating), increasing the mass fraction of sulfur in the positive electrode (typically the mass fraction is below 50%), and increasing the rate capability of the cell (target discharge rate is 1 C or higher). While some Li/S cells described in the literature fulfill some of the objectives for cycle life, specific energy, and specific power, none of these cells have been industrialized for mass-market applications due to deficiencies in one or more areas.
A particular concern is the fact that in many Li/S cells, the soluble polysulfides are free to migrate throughout the liquid electrolyte, and they may accumulate in parts of the cell where they are not particularly accessible for the reactions necessary to charge and discharge the cell. This can lead to capacity fade and loss of power capability over time. IN the worst case, these polysulfides can migrate to the negative electrode, where they are reduced upon reaction with the lithium metal anode. These reduced polysulfides can react with less reduced polsulfides in solution, leading to a self discharge of the cell until eventually solid sulfides may be formed at the anode surface. Usually these highly reduced products are not recoverable; hence the “polysulfide shuttle,” as it is known in the literature, ultimately results in capacity fade and potentially cell failure.
To avoid this shuttle effect, several researchers have explored the use of immobilizing electrolytes (either solid electrolytes or liquid electrolytes with very low polysulfide solubility) to prevent migration of the active cathode material. Others have attempted to confine sulfur within nanoporous structures, while still others have attempted to coat the materials with a Li-transparent material. A challenge associated with these approaches is that Li2S is electronically insulating and therefore must be restricted to small domain size (<100 nm) in order for the cell to be charged and discharged at practically relevant rates. One approach to increasing the conductivity of the Li2S, and therefore the battery's rate capability, is to dope the Li2S with another element.
Previously, doping Li2S or S with other elements in Li/S cells required in-situ doping during battery operation because the charging and discharging of the battery involved a phase transition from S to Li2S2 and Li2S via a polysulfide (Li2Sx, 3<=x<=8) intermediate that dissolves in the electrolyte. Hence, there was no straightforward method for introducing other dopants from solution during precipitation of the S or Li2S (or Li2S2).